There are many processes for the preparation of polycarbonates from monomers. A preferred method is a two-phase interfacial process in which phosgene is contacted with aromatic bisphenols and condensed to produce high molecular weight polycarbonates.
The two-phase interfacial process results in a reaction mixture that is an emulsion of organic and aqueous alkaline phases. In order to recover the polycarbonate product from the organic phase in which it is found, the reaction mixture is separated into its organic and aqueous alkaline phases, and the organic phase is further processed to remove undesirable impurities including electrolytes, catalysts, residues of chain terminators, and unreacted starting materials.
Such reaction mixtures contain polycarbonate polymers with molecular weights up to 40,000 grams per mole (g/mol) on a polycarbonate basis and brine present in an amount around one quarter by weight of the reaction mixture. Further, the reaction mixtures may exhibit a wide range of viscosities. In conventional processes, the initial separation of the aqueous alkaline phase from the organic phase is not complete, but leaves the organic phase containing some ten percent or more of an aqueous component. Further, the organic phase contains residual impurities such as catalysts, residues of chain terminators, and unreacted bisphenols. Thus, additional steps are needed to purify the organic phase.
For example, U.S. Pat. No. 4,338,429 describes removing alkali metal hydroxide, catalyst, and salts from the organic phase by repeated settling or centrifugation, with the addition of either aqueous acid or water in each step. The process is enhanced by the addition of shear energy and/or cationic emulsifying agents and dispersing agents to the aqueous acid step, or water-soluble organic anionic compounds to the water step.
Another method for purifying the organic phase is coalescence, as described in U.S. Pat. No. 4,316,009 and German Patent DE 19510061. The continuous process described therein involves washing and re-emulsification, followed by re-separating the two phases by conveying the emulsion through a layer of fibers that causes coalescence of the separate phases.
Another method for purifying the organic phase is to employ a series of centrifuges to remove the residual aqueous component by application of very high gravitational forces, as described in U.S. Pat. No. 5,260,418. In this process, the separated organic phase is further contacted with water and various aqueous solutions to bring the impurities into the aqueous component, which is removed and discarded.
Phase separation and purification processes employing centrifuges are time consuming and expensive to install and operate. Moreover, alternative purification methods such as extraction by countercurrent water washing and gravity separation of the two phases are much slower and less efficient. Accordingly, there is a need to provide a method for the separation and purification of reaction mixtures resulting from the two-phase interfacial process for making polycarbonate that is efficient and economical.